1. Field of the Invention
The field of the invention relates to oscillators and more particularly to voltage controlled oscillators.
2. Related Art
Voltage controlled oscillators (VCOs) are typically used in radio frequency transceivers as a mechanism for upconverting an information signal from baseband to a particular transmission frequency (i.e., channel) or for downconverting an information signal from a particular transmission channel to baseband. VCOs typically use tank circuits and operates under the influence of a variable capacitance provided by a varactor. Tank circuits are devices that resonate at frequencies determined by inductive and capacitive elements. Changing the values of the inductive or capacitive elements changes the resonant frequency of the tank circuit, thus changing the output frequency of a VCO relying upon the tank circuit.
The varactor of a VCO may be a diode or some other similar device. As is known, the capacitance across some diodes changes as a function of a reverse-bias voltage. Where the reverse-biased diode is coupled to an inductor, the resonant frequency of the tank circuit formed thereby can be independently controlled by the voltage across the diode.
While VCOs are effective, low voltage VCO designs, relying upon on-chip varactor diodes, typically suffer from performance impediments such as frequency centering errors and large variations in tuning constants. Frequency centering errors can be reduced by providing a large tuning sensitivity range, but this can contribute to phase noise. Because of the importance of VCOs in communications, a need exists for a better method of improving the tuning linearity and centering frequency error of VCOs.
A system is provided for controlling an output frequency of a dual-input voltage controlled oscillator relative to a reference frequency.
The method includes the step of coupling a first tune control signal to a first input of the dual-input voltage controlled oscillator. The method further includes the steps of dividing the output frequency of the dual-input voltage controlled oscillator by a value substantially equal to a second tune control signal, comparing a phase of the divided output frequency with a phase of the reference frequency and coupling a phase difference of the compared phases to a second input of the dual-input voltage controlled oscillator.
The apparatus includes a first tune control signal coupled to a first input of the dual-input voltage controlled oscillator and a frequency divider adapter to divide the output frequency of the dual-voltage controlled oscillator by a value substantially equal to a second tune control signal. The apparatus also includes a comparator adapted to compare a phase of the divided output frequency with a phase of the reference frequency and a phase detector adapted to couple a phase difference of the compared phases to a second input of the dual-input voltage controlled oscillator.
Other systems, methods, features and advantages of the invention will be or will become apparent to one of skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.